1. Field of the Invention
A system and method of water treatment using a venturi injector facilitates the removal of dissolved oxygen from water thereby reducing the population of undesirable aquatic organisms present in the water while inhibiting corrosion. The system and method of water treatment has particular utility for use in connection with a vessel by allowing the vessel to treat ballast water that is being transported from one port area to another, thereby limiting environmentally adverse effects, while inhibiting corrosion. The system and method of water treatment may have other uses, such as in oil production.
For instance, before a ship leaves a port empty, or partially loaded, it takes on water into ballast tanks to maintain stability and adjust buoyancy. In virtually every case, this ballast water will contain living organisms which are affected by levels of dissolved oxygen in the water. When the ship reaches its destination and prepares to load its cargo, it discharges this ballast water, thus introducing potentially invasive species to the aquatic environment of the destination port. Approximately 40,000 major cargo vessels carry billions of tons of ballast water around the world annually and are thus believed to be responsible for the introduction of hundreds of marine invasive species to non-native environments. The total cost of these invasions is indeterminate, but several estimates put it in the billions of dollars.
To address this issue, many national governments and state governments in the United States have passed regulations governing vessel ballast water management. The International Maritime Organization has proposed draft guidelines recommending treatment of ballast water. The United States Coast Guard is presently developing guidelines for potential future ballast water treatment requirements for vessels trading into ports in the United States.
The vast majority of the world's fleet of ships, including naval as well as commercial vessels are constructed of steel. Steel corrodes when exposed to oxygen and water. Corroded steel structures on a vessel decrease seaworthiness, and extensive measures are taken to avoid it, and to repair it. Estimates of the cost to protect against and repair corrosion on vessels runs into the billions of dollars annually worldwide.
One area in a ship where corrosion is of particular concern is in the ballast water tanks. For example, the largest oil tankers may have up to 15,000,000 gallons (57,000 tons) of ballast water capacity. Prolonged exposure of the ballast tank structure to water (often salt water) creates a condition conducive to rapid corrosion. At the time of this writing, the cost to paint ballast tanks is typically $5.00 to $10.00 per square foot while other estimates suggest that the cost to repair corroded areas are approximately $500 per square foot.
Thus, systems that treat water in order to eliminate aquatic organisms while providing corrosion inhibition in a time and cost-efficient manner are desirable. One form of eliminating aquatic organisms in ballast water is through the deoxygenation of the water as the water is taken from the surrounding waterways. The concentration of a solute gas in solution is directly proportional to the partial pressure of the gas above the solution. (This physical phenomenon is governed by Henry's Law, and the dissolved concentration can be calculated using the Henry's Law constant for that solute.) As such, when exposed to a stripping gas (such as nitrogen or other low-oxygen gas mixture), oxygen readily diffuses out of water, which contains between 6 to 10 parts per million (0.001 percent) of dissolved oxygen, in an effort to return to the mixture found in air, which is approximately 79 percent nitrogen and 21 percent oxygen. The use of nitrogen gas to remove the dissolved oxygen present in ballast water has been documented as offering an efficient and economically desirable means of treating ballast water while also providing corrosion inhibition effects. See MARIO N. TAMBURRI et al.: Ballast water deoxygenation can prevent aquatic introductions while reducing ship corrosion. Biolog. Conserv. (2002) 103: 331-341. The Henry's Law constants for a variety of potential stripping gases and mixtures thereof show that a variety of gases can be used to deoxygenate water.
Aboard a vessel, an efficient way to expose the dissolved oxygen in water to a stripping is to create micro-fine bubbles of gas in the water. Micro-fine stripping gas bubbles created in water have the capability of transferring dissolved oxygen from the water as the micro-fine bubbles float from the bottom to the top of a tank. A commonly recognized efficient, safe, and reliable way to create micro-fine bubbles is through the use of a venturi injector.
2. Description of the Prior Art
Water treatment, and more particularly, ballast water treatment, apparatuses and methods are desirable for allowing vessels to treat water that is being transported from one port area to another. Such treatment limits the environmentally hazardous effects that may result when the water is later released into an environment that is ecologically different from that in which the water was originally obtained.
The use of water treatment apparatuses and methods are known in the prior art. For example, U.S. Pat. No. 6,171,508 to Browning discloses a method and apparatus for killing microorganisms in ship ballast water. However, the Browning '508 patent does not use stripping gas to deoxygenate the ballast water and consequently fails to disclose any corrosion inhibition properties, and has further drawbacks of using a less efficient vacuum mechanism to remove the dissolved oxygen from the ballast water.
U.S. Pat. No. 6,125,778 to Rodden discloses ballast water treatment that treats ballast water using ozone. However, the Rodden '778 patent does not provide for corrosion, and fails to use the more efficient venturi-injector facilitated means of to treat the ballast water.
Similarly, U.S. Pat. No. 5,192,451 to Gill discloses a method for controlling zebra mussels in ship ballast tanks that treats ballast water with a water-soluble dialkyl diallyl quaternary ammonium polymer. However, the Gill '451 patent does not allow for water treatment without the occurrence of a chemical reaction, and does not provide for any corrosion inhibition properties.
Additionally, U.S. Pat. Nos. 5,376,282 and 5,578,116 to Chang both disclose the use of a vacuum and agitation to remove dissolved oxygen from water in order to inhibit the survival of zebra mussels. However, neither the '282 nor the '116 patent provides for a more efficient venturi injector-facilitated delivery of stripping gas to deoxygenate the ballast water and have the further disadvantages of failing to provide for corrosion inhibition effects during the removal of dissolved oxygen from the water.
U.S. Pat. No. 6,126,842 to Decker discloses a method of low-concentration ozone wastewater treatment that injects a gas mixture of a low-concentration of ozone gas in oxygen into a wastewater stream while mixing to provide a reduction in the wastewater pollutants. However, the Decker '842 patent, although providing an efficient ozone-based treatment system using a venturi injector, does not disclose the treatment of ballast water on a vessel nor does the '842 patent offer the even greater advantages obtained using an oxygen stripping gas such as increased efficiency and corrosion inhibition.
U.S. Pat. No. 6,274,052 to Hartwig discloses the ozonation of pool water that uses a series of venturi injectors for ozone delivery. However, the Hartwig '052 patent does not disclose the injection of an oxygen stripping gas into the water through venturi injectors in order to deoxygenate the water, and has the additional deficiency of failing to provide for any corrosion inhibition effects during the process described.
U.S. Pat. No. 4,246,111 to Savard discloses an apparatus is provided for treating wastewater biologically and clarifying the biologically treated water. However, the Savard '111 patent does not use an oxygen stripping gas for the deoxygenation of water, preferably but optionally ballast water, further fails to provide for corrosion inhibition.
Lastly, U.S. Pat. No. 3,676,983, to Nold discloses an apparatus and method for degassing a liquid using a vacuum chamber and agitation. However, the Nold '983 patent requires cavitation of the liquid and does not use a venturi injector to more efficiently enhance the degassing of the liquid.
While the above-described inventions fulfill their particular objectives and requirements, the aforementioned patents do not describe a system and method for water treatment which allows a vessel to treat ballast water that is being transported from one port area to another while providing corrosion inhibition.
The aforementioned patents and other water treatment systems and methods currently known in the art make no provisions for the treatment of water using an injector means to facilitate stripping gas deoygenation while also providing corrosion inhibition.